Two projects are described in this application. Both emphasize the relation of cell structure to epithelial transport. 1. The action of ADH: a scanning and transmission electron microscopie study. We have been able to apply current high-resolution techniques in both scanning and transmission electron microscopy (SEM and TEM) to the problem of the intracellular action of antidiuretic hormone (ADH). We will use both SEM and TEM to determine in detail the structure of the terminal web of the toad bladder epithelial cell, and the specific filamentous attachments of the cytoplasmic tubules to the luminal membrane and surrounding cytoskeleton. These tubules fuse to the membrane in response to ADH, and deliver water-conducting particles to the membrane. We have proposed a "tilting" mechanism for tubule fusion, and will seek evidence for this mechanism. We will employ tannic acid-fixed sections, resinless sections and sections coated with ultrathin metal films in our SEM and TEM sections. We will identify specific cytoskeletal components by immunocytochemistry at the light and EM level. We will isolate and characterize the water-conducting particles. We will use our techniques in a study of the mammalian kidney, emphasizing glomerular cytoskeletal structure. 2. Autoradiography of diffusible solutes. Refinements in our system of "dry" autoradiography permit the study of the transport of water-soluble solutes such as urea across epithelia, and the effects of hormones and inhibitory agents on transport. These studies will now be extended to two new problems: the paracellular transport of ions and non-electrolytes across "leaky" epithelia, and the active transport of iodide by the choroid plexus. Both projects bring new techniques to the problem of epithelial transport. There is increasing interest in fusion and exocytosis in such phenomena as neurotransmission and hormone secretion; the ADH system under investigation in this application provides one of the most clear-cut examples of the relationship of the cytoskeleton to fusion. In the human kidney, glomerular filtration appears to be greatly influenced by the contractile properties of the mesangial cell and other cellular elements surrounding the basement membrane; these may be altered in important ways in renal disease. Our autoradiographic studies show cellular transport in a new way, and add to our understanding of water and electrolyte movement.